Recently, light emitting diodes (LEDs) using a gallium nitride-based semiconductor (hereinafter, referred to as GaN LEDs) have been expected to be substituted for conventional light sources such as incandescent lamps, fluorescent lamps, and mercury lamps. Therefore, active research has been done on high-power GaN LEDs. In general, since a GaN LED film layer is grown on a sapphire substrate that is an insulator, the associated LED device has a horizontal structure. As a result, the conventional LED device has disadvantageously a high operation voltage and a low light output because current spreading resistance increases during a high power operation thereof. In addition, the conventional LED device has also disadvantageously poor thermal stability because heat generated during the operation thereof cannot be efficiently dissipated from the sapphire substrate.
In order to solve the aforementioned problems, a flip-chip packaging method has been proposed to implement a high-power GaN LED. Since light emits upwards through the sapphire substrate from an active layer of the GaN LED having a flip-chip structure, a thick p-type ohmic electrode can be substituted for a transparent electrode, so that the current spreading resistance can be reduced. At this time, a material constituting the p-type ohmic electrode should have low absorbance and high reflectance. Since metals such as Ag and Al having a reflectance of 90% or more have a low work function, these metals are not suitable for contact metals of the p-type GaN ohmic electrode. On the other hand, with respect to an InGaN LED device, research has rarely been done on a high-reflectance p-type ohmic electrode in comparison to a conventional p-type transparent electrode.
A recent research shows that, an Ni/Au transparent electrode on which Al and Ag reflective layers are deposited can have a reflectance of 70% or more in a blue light range (see Appl. Phys. Lett. vol. 83, p. 311 (2003)). However, there is a problem in that properties of the electrodes drastically deteriorate at a temperature of 100° C. or more.